The present invention relates to multipath transmission systems used for sending and receiving multiple signals between units which have a multiplex transmission function and are arranged in automobiles.
One such multipath transmission system comprises a plurality of multiplex transmission units having a multipath transmission function. Each of the multiplex transmission units is located at a predetermined position within automobiles. The multiplex transmission units serve to collectively control device groups. For example, one device group includes windshield wipers, power windows, door lock, etc., and another device group includes various meters, warning lamp, indicators, etc. The individual units are connected to a common multiplex transmission path or so-called bus. Therefore, the transmission of multiple signals between the multiplex transmission units is performed through the bus to control the device groups.
To this end, the aforementioned multiplex transmission units each include a sending/receiving device for sending and receiving the multiple signals toward or from the bus. The sending circuit of the sending/receiving device, as is disclosed in U.S. Pat. No. 4,429,384, for example, is a switching circuit having an npn transistor, a pnp transistor, and a bias resistor. This switching circuit is connected to a corresponding line of the bus, and serves to adjust the potential on the line to one of two predetermined values, for example, 0 or 5 V, when the transistor is turned on or off.
In a multiplex transmission unit which is not serving the multiple signal transmission, the transistor of the sending circuit thereof is turned off. Thus, the multiple signals delivered from any other multiplex transmission units onto the bus cannot be destroyed.
It is to be first noted, with respect to the multipath transmission system described above, that if the number of multiplex transmission units connected to the bus increases, then the number of bias resistors connected in parallel with each other to one line of the bus also increases, so that the current of the transistor in the sending circuit of one transmission unit varies, depending on the number of units connected to the bus, when the multiple signals are transmitted from the sending circuit of one transmission unit to the bus line. Secondly, the increase of the bias resistors in number results in too small a resistance value for the whole circuit associated with the bus line. Accordingly, the current of the transistors in the sending circuits may possibly exceed its maximum permissible level, thereby damaging the transistors.
The multipath transmission system may be provided additionally with a limiter circuit for limiting the current of the transistors. In this case, however, the signal levels produced on the bus becomes too small to transmit the multiple signals.
In incorporating the multipath transmission system in an automobile, therefore, the aforementioned circumstances should be taken into consideration. More specifically, automobiles of the same model may be furnished with different meters or devices, that is, they may be different in grade. As regards the multiplex transmission unit, the difference in grade may possibly result in variation of the number of multiplex transmission units connected to the bus. If the number of transmission units varies, then the current of the transistors in the sending circuits changes considerably during the multiple signal transmission, as mentioned before. Such a change of the current constitutes a hindrance to secure multiple signal transmission. In actually incorporating the multipath transmission system in the automobile, therefore, it is necessary to change the circuit configuration of the multipath transmission system in accordance with the variation of the number of multiplex transmission units attributable to the difference in grade.
In order to minimize power consumption in the automobile, moreover, a system is employed in which power circuits for the individual devices are opened or closed in accordance with the key position of an ignition switch. Also in the multipath transmission system, therefore, it is advisable to use a system in which power circuits for the multiplex transmission units are opened or closed in accordance with the key position. In the multipath transmission system described above, however, even though there are multiplex transmission units whose power circuits are opened for non-operating, the bias resistors of the sending circuits of these units are left connected to the bus lines. Thus, the current flowing through the transistors of the sending circuits may vary, depending on the number of multiplex transmission units whose power circuits are closed. This may possibly lead to a change of voltage on the bus, so that the signals cannot be accurately transmitted through the bus.